1. Field of the Invention
This invention relates to the field of distance measuring receivers. More particularly, this invention relates to a distance measuring receiver which requires a single signal to be transmitted from a fixed location.
2. Related Art
It would be desirable for many reasons to provide an extremely accurate positioning system, specifically one for use in nautical navigation and mapping. It will be appreciated that the location of such things as oil exploration and drilling rigs is extremely critical since an error on the order of meters can have a devastating effect on the success or failure of a particular exploration. Similarly, for navigational purposes, extremely accurate positioning determination of the location of receivers from a remote transmitter is desirable.
The related art includes a number of systems which propose to provide position measurement information by measurement of the phase difference between two continuous wave signals. In U.S. Pat. No. 4,283,726, entitled "Dual Frequency Distance Measuring System and Method," to Spence shows a novel transmitter for a dual signal distance and position measuring system, which patent is incorporated by reference herein. In the conventional two signal system, two continuous wave signals precisely displaced in frequency are transmitted from a fixed transmitting station to a mobile receiving station.
At the receiving station, the two received signals are mixed together to produce a difference frequency or delta signal having a frequency equal to the frequency displacement, which defines the length of a "coarse" lane in which the receiver is located. The phase of the delta frequency signal is compared to the phase of a reference signal generated at the receiver. The frequency of the reference signal is the same as the delta signal, and the phase is identical to the phase of the reference signal generated by a source at the transmitting station used to produce the transmitted pair of signals. The difference between the phase of the delta signal and the phase of the reference signal generated at the receiver is indicative of the relative position of the receiver in the coarse lane.
The position of the receiver within a "fine" lane is determined by comparing the phase of one of the received signals with the phase of a signal of the same frequency generated by the source at the receiver. Under normal operating conditions, however, it is impossible to accurately determine the fine lane in which the receiver is located because the resolution of the coarse lane phase comparision is not high enough.
The applicant, in a subsequent invention disclosed in U.S. patent application Ser. No. 543,602, entitled "Distance Measuring Receiver System and Method," incorporated by reference herein, invented a dual signal receiver which creates in real time lanes of intermediate length between the coarse and fine lanes. These intermediate lanes are created from the two received signals by frequency multiplying them in linear expansion amplifiers which exhibit phase integrity. The resolution of the phase comparison in each of these intermediate lanes is relatively the same as that obtained for the fine and coarse lane measurements. However, the different lengths of the intermediate lanes allows this receiver to zero in on the fine lane in which the receiver is located.
The applicant has also recently invented a constant number frequency generator which may be used to replace the atomic clocks required by the conventional distance measuring systems. This invention is described in U.S. patent application Ser. No. 835,295, entitled "Constant Frequency Signal Generator Circuit and Method," filed on Mar. 3, 1986 U.S. Pat. No. 4,692,715 which application is incorporated herein by reference.